Motor control system



y 0, 1940. c. P. SWEENY 2,209,364

MOTOR CONTROL SYSTEM,

Filed Nov. 14, 1938 naw as Ma Zhwwrvtom Patented July 30, 1940 PATENT OFFlCE MOTOR c'oN'raoL SYSTEM Charles P. Sweeny, Detrolt,,Mich., assi'gnor to Vickers, -Incorporated, Detroit, Mich a corporation of Michigan Application November 14, 19 38, Serial No. 240,425

7 7 Claims.

This invention relates to motor control systems and more particularly to systems for controlling and stabilizing the operation of variable speed alternating current motors having direct cur ent energization.

A variable speed alternating current motor, the speed of ,whichcan be varied by varying the direct current energization of certain windings of the motor, is disclosed in my co-pending application, Serial No. 72,632, filed April 3, 1936. This motor contains a primary alternating current winding upon one member of the motor, a secondary winding upon the other member of the motor and a supplemental control winding preferably positioned upon the member which supports the primary winding. The motor is essentially an induction motor in which a low power, direct current energization is applied to the control and secondary windings to provide for speed variation by varying the degree of such energization. In the motor referred to, increase of direct current energization decreases the speed of the motor and decrease of direct current energization increases the speed of the motor. The motor has been found to have an extremely wide speed variation and good regulation. That is tosay, the speed does not vary widely under changing load conditions orv changing line voltage conditions. There is, however, some variation of speed when load and voltage conditions change and the presentinvention is directed to a control system in which such speed variations are sub: stantially entirely eliminated.

It is, therefore, an object of the present invention to provide a control system for improving the speed regulation of variable speed alternating current motors.

Another object of the invention is to provide a motor control system for alternating current motors having direct current energization for varying the speed thereof, which control system minimizes speedvariation under varying load and voltage conditions.

Another object of the invention is to provide a motor control system in which the speed of an alternating current motor is charged by changing the direct current energization applied to the motor and in which the voltage oi the direct current energization is automatically varied with changes in line voltage to maintain the speed of the motor substantially constant.

Another object of the invention is to provide a motor control system in'which the speed of an alternating current motor having direct current energization is maintained substantially constant under varying voltage and load conditions by automatically varying the' voltage of the direct current energization when the line voltage or load varies.

A further object of the invention is to provide 5 a motor and control system in which the speed of an alternating current motor is varied by varying direct current energization of the motor and in which voltages induced in a secondary winding of the motor are employed to vary the direct 10 current energization of the motor in order to maintain the speed of the motor. substantially constant under varying loads.

Other objects and advantages of the invention will appear in the following description of pre- 18 ferred embodiments of the invention disclosed in the attached drawing, of which:

Fig. 1 is a schematic diagram of a motor control system of the present invention, and

Fig.2 is a similar diagram of a modified motor 20 control system.

Referring more particularly to the drawing, A indicates a primary winding of the variable speed alternating current motor, which primary winding is energized from an alternating current 25 source of power through the conductors A1, A: and A3. The winding A is shown as a three phase winding although other polyphase wind ings may be employed and even a single phase winding may be employed if supplemental start- 30 ing coils or mechanism for the motor are provided. The winding A is ordinarily positioned upon the stator of the motor although obviously it may be positioned upon the rotor. A control winding B is preferably positioned upon the same member as the winding A and is energized from a source of direct current hereinafter described through the conductors B1 and B2. The A and B windings are preferably distributed windings occupying the same slots of the member upon which they are positioned and'the conductors of the B winding are so positioned and connected that no resultant voltages appear across the conductors B1 and B: when the A winding alone is energized. This can be done, for example, by employing a different number of poles for the B winding than for the A winding or in case the B winding has the same number of poles as the A winding, by positioning and connecting the coils of the Bwinding so that the voltages in- 50 duced therein from the flux produced by the energization of the A winding oppose each other. In either case the'conductors of the B winding are so positioned and connected that the B winding does notform a short circuit path for cur- 55 stantial control over the amount of current after rents produced therein by induction from the A winding when-the A winding alone is energized. Ordinarily, both the A and B windings are connected to produce four or niorepolesand as an example, the B winding may be connected to produce six poles, while A winding is connected to produce eight poles.

. The secondary winding C is positioned on the other member of the motor and preferably upon the rotor. This winding is energized from the source of direct current above mentioned through conductors C1 and C2 and is preferably connected to produce two poles. A conventional two-pole distributed winding may be employed as V a C winding but a better flux distribution is secured by employing-a plurality of concentric coils dis= posed in slots for each pole. The greater the energization of B and C windings with-direct current the slower the speed of the motor. D. C. energization necessary to effect a wide range of speed control is small with comparison to the A. C. energization of the A winding such that the motor is essentially a variable speed alternating current motor. The voltage applied to the A winding for normal operation of the motor is sufiiciently low that the iron in the motor is subnormally saturated and the power currents in the C winding are relatively small so that the power produced is relatively smallin the absence of D. C. energization. Under these conditions, D. C. energization of either the B or C windings or both increases the saturation of the iron and distorts the flux from the A winding so that increased amounts thereof cut the conductors of the C winding. The greater the D. C. energization, the greater the power currents, in the C winding and the slower the speed of the motor the greater the amount of power produced by the motor. v

The D. C. energization for the motor is in accordancewith the present invention derived from the same source of power as the A. C. energization. Rectifying devices shown as rectifying tubes it and H furnish the D. C. energization for the motor. These tubes are preferably gas filled power tubes containing heated cathodes I 2, plates l3 and grids l4. Such tubes have substantially a-constant voltage drop irrespective of the amount of current passing therethrough and the grids of the tubes will control the time of starting of current flow through the tubes but have no subcurrent flow has started. Since the current through either tube ID or H is zero for alternate half cycles of the alternating current power applied thereto, the grids are effective to control the time during which current flows through the tubes and thus the total amount of current. The tubes ll and II are supplied with plate voltage from a plate transformer l5 receiving power from conductors I6 and I1 connected across conductors A and A forming one phase of thealternating current source of power. An auto transformer coil 13 forming part of a control reactor is is connected across the conductors l6 and H. The primary winding 20 of theplate transformer I! has one side connected to the conductor l6 and the other side connected to a variable tap 2| of the auto. transformer coil l3. Thus, the primary 23 ofthe transformer I! can be supplied with an alternating current voltage which can be variedby varying the position of the tap 2| of the auto transformer coil I8. The plates l3 of the tubes l and II are connected to opposite terminals of the center tapped secondary 22 of The the transformer l8. Heating current is supplied to the cathodes I2 from a center tapped secondary 23 of a filament transformer 24 having a primary 25 connected across the conductors l6 and I7. The center tap of the secondary 23' of the filament transformer 24 is connected to the'conductor C2 by a conductor 26 and the path of direct current from the rectifier tubes lb andll can be traced from the cathodes I? through the center tap of the secondary 23, conductor 26 and conductor C2 "through the-B and C windings and then through the conductor Cl, coil 21 of the control reactor is and conductor is to the center tapof the secondary 22 of the plate transformer It. The control reactor also has a coil 23 connected between the center taps of the secondaries 22 and 23 ofthe transformers l and 26 by means of conductors 28 and 25 for purposes which will hereinafter appear.

The control reactor is has the auto transformer coil l8, the coil 2? and the coil 28 all mounted upon a single iron core. The coils 2i and 28 are pref= erably connected so that voltages induced therein by the flux produced by the auto transformer coil it oppose each other. The iron of the core of this reactor is operated at relatively high saturation so that there is substantial leakage flux between the turns of the auto transformer coil 68. The saturation of the iron of this device is maintained relatively high by the coil 28 connected under load. These larger currents increase the saturation of the iron'of the control reactor l9 to still further increase the leakage reactance of auto transformer coil I! so as to decrease the voltage applied to the tubes I6 and I2 and thus the direct current energization of the-windings B and C. Thus, as the speed of the motor tends to decrease under load, the D. C. energization of the windings B and C is decreased to prevent substantial decrease in speed of the motor under load. The normal speed of the motor can be set by moving the adjustable tap 2! of the auto transformer coil l8 to produce the desired motor speed and the changing saturation of the core of the control reactor is will maintain the set speed of the motor substantially constant under varying load.

A half wave rectifier shown as a tube 30 is employed in combination with the tubes Ill and H to maintain the speed of themotor substantially constant under conditions of varying voltage supplied by the alternating current source of power. This tube is preferably of the vacuum type, the output voltage of which is substantially proportional to the alternating input voltage. A

' filament transformer 3| having a center tapped secondary 32 and having a primary 32' connected across the conductors l5 and i1 may be employed to supply heating current to the cathode 33 of the tube 39. A plate transformer 33 having a primary 34 connected across the conductors l1 and I6 and a secondary 3 5 having one side connected to the center tap of the secondary 32 of the filament transformer 3| and the other side connected to the plate 36 of the tube30 through resistors 31 and 38 may be employed to supply plate voltage for tube 30. The end 39 of the resistor 38 remote from the plate 35 is connected to the center tap of the secondary 23 of the filament transformer 24 of the tubes Ill and il through a conductor 40 and resistor 4| in order to tie the plate circuit of the tube 30 to the cathodes of the tubes I0 and II. The grids l4 of the tubes l0 and II are connected to a slider 42 engaging resistor 31. Direct current due to rectification in the tube 30 will flow through resistor 31 so as to tend to hold the grids l4'of the tubes Ill and H positive withrespectto the cathodes of these tubesdue to I. R. drop in the portion-of resistor 31 between the connection 33 and the slider 42.

.Since the tube 30 is a half wave rectifier,-the

cluding a conductor 43 from-one side of the sec 31 and an alternating current potential is also superimposed on the pulsating direct current applied to the grid and tubes l4. The tubes ill and H are preferably of the type in which no substantial current flows through thetubes unless the grids l4 have a positive bias applied thereto. This positive bias is supplied by the current flowing through the portion of the resistor 31 between connection 39 and slider 42. The characteristics of the tube 30 are such that this bias is increased at a greater rate than an increase of input voltage applied to the tube 33. Thus, upon increase in voltage from the alternating current source, which increase would tend to cause the motor to increase its speed, an increased bias is applied to the grids of the tubes I0 and l I to increase the direct current energization of the windings B and C, which increased energization tends to decrease the speed of the motor such that the two effects compensate and the motor remains at substantially constant speed. The reverse effect occurs when the line voltage drops as the D. C. energization is decreased at a greater 'rate than the decrease in line voltage such that the speed of the motor remains substantially constant. Since the grids of the tubes Ill and II have a positive potential applied thereto, these grids take current which returns to the cathode of the tubes through resistor 48 and conductor 26. The I. R. drop in the resistor 44 is in a direction to decrease the positive potential of the tubes so that the positive potential applied by the resistor 37 is in part compensated to stabilize the operation of the device.

In operation, tubes Ill and H provide D. C. energization for the windings B and C of the motor, which D. C. energization, and thus the speed of the motor, can be varied by moving the variable tap 2i of the auto transformer coil l8. This D. C. energization then remains substantially constant under constant load and line of the motor substantially constant under varying load conditions. If the line voltage varies, thereby tending to vary the speed 011 the motor, the

tube 30 causes the positive potential applied to the grids l4 of the'tubes l0 and ii to increase or decrease faster than the increase or decrease, respectively, of line voltage so that the D. C. energization of the windings B and C increases or decreases at a faster rate than the increase or decrease, respectively, of the line voltage; the speed of the motor is thereby retained substantially constant under varying voltage conditions. The modification shown in Fig. 2 is similar in many respects to the modification shown in Fig. 1 and produces substantially the same results. The same reference characters have been employed in this modification where the elements are the same as those in Fig. 1. In the system of this figure, no control reactor is employed and a simple auto transformer or other voltage varying device 45 is employed to vary the voltage applied across the plates and cathodes of the tubes I0 and H. The D. C. energization of the windings B and C can be varied by moving the adjustable tap 2| of the auto transformer winding il in order to vary the speed of the motor. The tube 3! controlling the bias of the grids l4 and tubes I0 and II may be substantially the same as the tube 30 of Fig. 1 but is connected to the tubes l0 and Ii in a somewhat diflerent manner. Tube 30' provides half wave rectification and the pulsating direct current produced thereby flows from the center tap of the secondary winding 32 of the filament transformer 3| through the secondary winding 35 of the plate transformer 33 and then through resistors 46 and 41 to the plates 36' of the tube 30. The plates 36' of the tube 30' are tied to the cathodes l2 of the tubes i0 and it through a conductor 48 and a resistor shown as a lamp 49 which has a high positive temperature coefficient of resistance. The grids It of the tubes If) and ii are connected to the end of the potential of the grids i4 can be adjusted by moving the slider 5i of the resistor 48. The tube 30' varies the positive bias of the grids M of the tubes In and H as the line voltage varies so as to cause the direct current energization of the windings B and C to be increased or decreased at a greater rate than the increase or decrease, respectively, of the line voltage in order to maintain the speed of the motor substantially constant under conditions of varying, line voltage. Grid current flowing through the resistance of lamp 4Q tends to decrease this positive potential in order to provide for partial compensation of the positive grid bias to render the system stable. Also, grid current flowing through a high resistant 52 connected between the grids and the cathodes it has a similar compensating action stabilizing operation of the system.

The lamp 43 and also a similar lamp 53 connected across the conductors C1 and C2 in series with the lamp 43 are employed to main tain the speed of the motor substantially constant under varying load conditions and perform substantially the same function as the control reactors IQ of Fig. 1. It has been found that increased load upon the motor tending to slow the motor down causes increased voltages in the secondary winding C: These voltages are applied across lamps 49 and 53 in series and cause increased current flow through the lamps 49 and 53 when the load of the motor increases. This increases 75 the temperature 01' the filaments of the lamps which have a high positive temperature coemcient resistance to increase the resistance of the lamp Q9. The grid current of the tubes is and ii flows through the lamp 49 such that the positive bias on the grids is decreased as the temperature of the filament of lamp 89 increases. Thus, as load upon the motor is increased, the positive bias of the grids i i of the tubes it and ii is decreased to decrease the output of these tubes and therefore decrease the direct current energizetion of the windings B and C. This decreased energization prevents substantial decrease of the speed of the motor. The opposite efiect takes place when load on the motor decreases, such that the speed of the motor is maintained substantially constant under varying loads. Thus the motor control system of Fig. 2 as well as that of Fig, 1 maintains the speed of a variable speed alternating current motor having D. C. energize.- tion, substantially constant under both varying load and varying line voltage conditions. 7

While I have disclosed the preferred embodiments of my invention, it is understood that I am not to be limited to the precise disclosure thereof, but the details may be varied within the scope of the following claims:

What I claim is:

1'. In a motor control system, a source of alternating current power, a rectifying device receiving power from said source and constituting a source of direct current power, an alternating current motor of the induction type having relatively rotatable members a primary winding upon one'oi said members energized from said source of alternating current power, a secondary winding upon the other of said memberaand a control winding upon said one of said members and energized from said source of direct current power ior varying the normal speed of said motor inversely to the amount of D. C. energization of said secondary winding, a second rectifying device receiving power from said alternating current source of power for controlling said first rectifying device, to increase or decrease the direct current energization of said control winding at a greater rate than the increase or decrease,

' respectively, of voltage of said alternating source of power, in order to stabilize the speed of said motorunder varying voltage conditions of said- 'alternating current source oi power.

2. Inc motor control system for stabilizing the speed of a variable speed alternating current motor of the induction time under variable voltage conditions applied thereto, an alternating current source of power, means receiving energy from said source oi. power for converting alternating current into direct current, a variable speed alternating current motor of the induction type having relatively rotatable members a primary winding upon one of said members and energized from said source of alternating current power, a secondary winding upon the, other. of said members, and a control winding upon said one 0! said members-and energized from said means to vary the speed of the motor inversely to the direct current energization supp ed by said said alternating current source of power, in order to maintain the speed of said motorsubstanaaoassa tially constant under varying voltage conditions of said alternating source of power.

3. In a motor control system for maintaining the speed of a variable speed alternating current motor'substantially constant under varying voltage conditions of the alternating current power applied to said motor, which comprises, an alternating current source of power, a direct current source of power receiving alternating current power iromsaid alternating current source of power and converting the same into direct current power, a variable speed alternating current motor having relatively rotatable members a primary winding upon one of said members and energized from said source of alternating current power, a secondary winding upon the other of said members and a control winding upon said one of said members and energized from said direct current source of power to vary the speed of said motor inversely to the direct current energization of said secondary winding, means responsive to the voltage of said alternating current source of power for controlling said direct current source of power so as to vary the direct current energization of said control winding to maintain the speed of said motor substantially constant under varying voltage conditions of said alternating current source of power.

4. In a motor control system'for stabilizing the speed of an alternating current motor of the type which varies its speed under variable voltternating current motor having relatively rotatage conditions of the alternating current power able members a primary winding upon one of said members and energized from said alternating current source of power, a secondary winding upon the other of said members, and a control winding upon said one of said members and energized from said direct current source of power to vary the speed of said motor as the direct current energization of said last-mentioned winding is varied, manually actuated means for varying the output of said direct current source speed of an alternating current motor of the.

type which varies its speed under variable voltage conditions of the alternating current power applied thereto, which comprises, an alternating current source of powe a direct current source of power receiving alternating current powerv from said alternating current source of power and converting the same to said direct power, an alternating current motor having relatively rotatable members a primary winding upon one ofsaid members and energized from said alternating current source or power, a secondary winding upon the other of said members, and a control winding upon said one 01'. said members and energired -from said direct current source of power to vary the speed of saidmotor as the direct current energization of said last-mentioned winding is varied, means responsive to voltages induced in said secondary winding for varying the output of the direct current source oi power to co" spnsate for variations in motor speed under vs :1- ing load conditions, andmeans responsive to the voltage oi said alternating-current source of power for varying the output or said direct current source of power as the voltage of said alternating currentsource of power varies .to maintain the speed of said motor substantially constant under va y Voltage conditions of sal alternating current source or power. I

6. In a motor control system for stabilizing the speed of an alternating current motor of the type which varies its speed under'variable voltage conditions of the alternating current power applied thereto, which comprises, an alternating current source of power, a direct current source or power" receiving alternating current power irom said alternating current source of power and converting the same to said dir ct power, an alternating current motor having relatively rotatable members a primary winding upon one of said members and energized from said alternating current source of power, a secondary winding upon the other or said members, and a control winding upon said one of said members and energized from saidjdirect current source of power to vary the speed of said motor as the direct current energization or said last-mentioned winding is varied, means including a saturable core transiormer, responsive to voltages induced in said secondary winding for varying the output of the direct current source of power to compensate for variations in motor-,speed under varying load conditions, and means responsive to the-voltage or said alternating current source of power tor varying the output of said direct current source oi. power as the voltage of said alternating curreceiving alternating current power from said alternating current source of power and converting the same to said direct power, an alternating current motor having relatively rotatable members a prlmarywinding upon one of said mem-\ bers and energized from said alternating current source of power, a secondary winding upon the other or said members, and a control winding upon said one of said members and energized from said direct current source of power to vary the speed of said motor as the direct current energlzation of said last-mentioned winding is varied, means including a heated resistor having a high positive temperature coefllclent of resist- .,ance, responsive tofvoltages induced in said second winding for varying the output of the direct currentsource of power to compensate for variations in motor speed under varying load conditions, and means responsive to the voltageof said alternating current source of power for varying the output of said direct current source of power as the voltage of said alternating current source of power varies to maintain the speed of said motor substantially-constant under varying voltage conditions of said alternating current source of power.

' CHARLES P. SWEENY. 

